A conventional optical biometric system can be used to detect and recognize faces, voices, irises, retinas or fingerprints. For example, in an optical fingerprint recognition system, a conventional image capture apparatus includes at least a luminous element, a transparent element and an image sensor, wherein the transparent element is disposed upon the image sensor.
After being generated by the luminous element and transmitted to the transparent element, a light beam is totally reflected at an interface between the transparent element and an environmental medium and is transmitted to the image sensor. When users put their fingers on the transparent element, fingerprint patterns captured by the image sensor have dark and light stripes, respectively corresponding to ridges and valleys. Then, an image processing apparatus processes the fingerprint patterns captured by the image sensor and the users' identities can further be determined.
In prior art, the image sensor includes a plurality of pixel areas, respectively receiving light beams respectively reflected by different areas of the transparent element. In order to prevent the pixel areas from receiving stray light to interfere with imaging quality when an intense environmental beam (e.g., solar radiation) radiates, the conventional image capture apparatus further includes a light limiting element disposed between the transparent element and the image sensor. Specifically, FIG. 1A is a partial top view of the conventional light limiting element disposed on the image sensor. In ideal circumstances, a light limiting element 12 has a plurality of openings respectively aligned with a plurality of pixel areas 110 to limit an amount of incident light of each of the pixel areas 110.
However, alignment accuracy between the light limiting element 12 and an image sensor 11 may be more demanding. FIG. 1B is a partial top view of the conventional light limiting element 12 disposed on the image sensor 11. If the alignment between the light limiting element 12 and the image sensor 11 is inaccurate, an opening 120 of the light limiting element 12 will be deviated relative to the corresponding pixel area 110. That is to say, the opening 120 of the light limiting element 12 and the corresponding pixel area 110 can partially overlap merely in a vertical direction and cannot completely overlap. Therefore, a light beam passing through the opening 120 cannot be completely received by the corresponding pixel area 110, such that an overall amount of incident light of the image sensor 11 will greatly reduce and the imaging quality will be affected.